Currently, there is substantial interest in three-dimensional (3D) transistor structures such as multi-gate FinFETs and 3D memory devices. Examples of some 3D structures can be found in “Tri-gate,” J. Kavalieros et al. (Intel), Symp. VLSI Tech, p. 50 (2006), and “Opportunities and Challenges of FinFET as a Device Structure Candidate for 14 nm Node CMOS Technology,” ECS Transactions, Vol. 34, Issue 1, pp. 81-86 (2011), both hereby incorporated by reference herein in their entireties and for all purposes. Advanced 3D gate structures are being employed in the 22 nm technology node, and likely in further technology nodes having features sizes below 22 nm. Transistors with 3D gate structures typically employ source and drain regions formed in thin vertical structures. These vertical structures are difficult to dope using conventional ion implantation techniques, plasma doping techniques, and generally, techniques which involve transport of ions under an electric field. The difficulty is generally manifest when doping sidewalls, and especially manifest in high aspect ratio structures.
Thus, since ion implantation processes are largely directional, they tend to be incompatible with the fabrication of many gate architecture designs, such as 3D gate designs and/or gate architectures having closely spaced fins which may require doping on lateral and vertical surfaces. If attempt is made to use ion implantation for doping these surfaces such as by changing the implant tilt angle, this may lead to considerable variability in the dopant dose retention and diffusion range, especially on the sidewalls. Frequently, in a dense array of i3D structures, there can be shadowing effects for the directional ion beam in an implanter, giving rise to serious dose retention problems for tilted implant angles. There has also been recent interest in the plasma doping technique. However, there appear to be two primary disadvantages to this technique: (a) it may lead to simultaneous sputter erosion, which may be due to the high energy ions in the plasma, and (b) dopant dose and conformality seem to sensitively depend on the ion to radical density ratio in the plasma, which leads to difficulties in process control. Thus, there is an ongoing need for better doping methods which may be used to fabricate many current gate architecture designs.